Symmetric serrated edge light guide having circular base segments

ABSTRACT

The present invention provides a planar light guide film for a backlight unit having at least one point light source, the light guide film comprising a light input surface for receiving light from the point light source, a light redirecting surface for redirecting light received from the light input surface and a light output surface for outputting at least the light redirected from the light redirecting surface. The light input surface further comprises a composite lens structure having a circular tip segment with a first contact angle, and a first and second circular base segments with a second contact angle, the second contact angle being greater than the first contact angle and the second contact angle being equal to each other. Further, the circular tip segment satisfies the following equation: 
         y   1   =a   1 +√{square root over (( r   1   2   −x   2 ))}
 
     and the circular base segments satisfies the following equations: 
         y   2   =b   2 +√{square root over (( r   2   2 −( x+a   2 ) 2 ))}
 
         y   3   =b   3 +√{square root over (( r   3   2 −( x−a   2 ) 2 ))}

FIELD OF THE INVENTION

The present invention relates to a light guide film of a light emittingdiode (LED) backlight unit, and, more particularly, to a light guidefilm of an LED backlight unit, which has a plurality of grooves carvedinto an incident plane of the light guide film to increase an incidenceangle of which light can be transmitted through the light guide film.

BACKGROUND OF THE INVENTION

Typically, a liquid crystal display (LCD) for handheld and notebookdevices generally employs at least one lateral light emitting diode(LED) as a light source of a backlight unit. Such a lateral LED isgenerally provided to the backlight unit as shown in FIG. 1 of Yang U.S.Pat. No. 7,350,598.

Referring to FIG. 1, the backlight unit 10 comprises a planar lightguide film 20 disposed on a substrate 12, and a plurality of lateralLEDs 30 (only one lateral LED is shown in FIG. 1) disposed in an arrayon a lateral side of the light guide film 20. Light L entering the lightguide film 20 from the LED 30 is reflected upwardly by a minutereflection pattern 22 and a reflection sheet (not shown) positioned onthe bottom of the light guide film 20, and exits from the light guidefilm 20, providing back light to an LCD panel 40 above the light guidefilm 20. Such a backlight unit 20 suffers from a problem as shown inFIG. 2 when light is incident on the light guide film 20 from the LED30.

As shown in FIG. 2, light L emitted from each LED 30 is refracted towardthe light guide film 20 by a predetermined angle θ due to difference inrefractive index between media according to Snell's Law when the light Lenters the light guide film 20. In other words, even though the light Lis emitted at a beam angle of α1 from the LED 30, it is incident on thelight guide film 20 at an incidence angle of α2 less than α1. In FIG. 3,such an incidence profile of light L is shown. Therefore, there is aproblem of increasing the length (l) of a combined region where beams oflight L entered the light guide film 20 from the respective LEDs 30 arecombined. In addition, light spots H also called “hot spots” and darkspots D are alternately formed in the region corresponding to the length(l) on the incident plane of the light guide film 20. Each of the lightspots H is formed at a location facing the LED 30, and each of the darkspots D is formed between the light spots H.

Since the alternately formed light and dark spots are not desirable forthe light guide film, they should be minimized and the length (l) shouldbe shortened as much as possible. For this purpose, it is necessary toincrease an angle of light entering the light guide film, that is, anincidence angle of light.

For this purpose, it is suggested to form protrusions on the inputsurface of the light guide film as shown in FIG. 4. Specifically, aplurality of fine prism-shaped structures 24 or arc-shaped structures(not shown) are formed on a light input surface 20A of a light guidefilm 20 and light L enters the light guide film at an incidence angle α3substantially equal to an orientation angle α1 of light emitted from afocal point F of a light source. Thus, if orientation angles α1 of lightbeams emitted from the focal point F of the light source are identical,the light L enters the light guide film at an incidence angle α3 widerthan the case of FIGS. 2 and 3. However, with this solution, there issome secondary light collimation where the light rays are refracted bythe wall of the adjacent prism or arc-shaped structure as shown in FIG.4. Secondary light collimation from the walls of the adjacent prismstructure turns the light ray back on-axis providing less diffusion ofthe light from the light source as shown in FIG. 4. Thus the continuousprism- or arc-shaped structures on the input surface have limited lightdiffusing capability.

Therefore an improved input edge design is needed to provide a moreuniform surface illumination of the light guide film without sacrificingthe efficiency of the backlight system.

SUMMARY OF THE INVENTION

The present invention is aimed at overcoming the problems (hot spots andsecondary light collimation) associated with the above prior art andtherefore yield a more uniform surface illumination of the light guidefilm without sacrificing the efficiency of the backlight system.

The present invention provides a planar light guide film for a backlightunit having at least one point light source, the light guide filmcomprising: a light input surface for receiving light from the pointlight source; a light redirecting surface for redirecting light receivedfrom the light input surface; a light output surface for outputting atleast the light redirected from the light redirecting surface; whereinthe light input surface further comprises a composite lens structurehaving a circular tip segment with a first contact angle, and a firstand second circular base segments with a second contact angle, thesecond contact angle being greater than the first contact angle and thesecond contact angle being equal to each other; and wherein the circulartip segment satisfies the following equation:

y ₁ =a ₁+√{square root over ((r ₁ ² −x ²))}

and the circular base segments satisfies the following equations:

y ₂ =b ₂+√{square root over ((r ₂ ²−(x+a ₂)²))}

y ₃ =b ₃+√{square root over ((r ₃ ²−(x−a ₂)²))}

In addition, the invention further provides a planar light guide filmfor a backlight unit having at least one point light source, the lightguide film comprising: a light input surface for receiving light fromthe point light source; a light redirecting surface for redirectinglight received from the light input surface; a light output surface foroutputting at least the light redirected from the light redirectingsurface; wherein the light input surface further comprises a compositelens structure having gaps there between, the lens structure having acircular tip segment with a first contact angle, and a first and secondcircular base segments with a second contact angle, the second contactangle being greater than the first contact angle and the second contactangle being equal to each other; and wherein the circular tip segmentsatisfies the following equation:

y ₁ =a ₁+√{square root over ((r ₁ ² −x ²))}

and the circular base segments satisfies the following equations:

y ₂ =b ₂+√{square root over ((r ₂ ²−(x+a ₂)²))}

y ₃ =b ₃+√{square root over ((r ₃ ²−(x−a ₂)²))}

Further, the invention provides a planar light guide film for abacklight unit having at least one point light source, the light guidefilm comprising: a light input surface for receiving light from thepoint light source; a light redirecting surface for redirecting lightreceived from the light input surface; a light output surface foroutputting at least the light redirected from the light redirectingsurface; wherein the light input surface further comprises a serratedlens structure that is provided only where the point light source isincident on the light input surface, the lens structure having acircular tip segment with a first contact angle, and a first and secondcircular base segments with a second contact angle, the second contactangle being greater than the first contact angle and the second contactangle being equal to each other; and wherein the circular tip segmentsatisfies the following equation:

y ₁ =a ₁+√{square root over ((r ₁ ² −x ²))}

and the circular base segments satisfies the following equations:

y ₂ =b ₂+√{square root over ((r ₂ ²−(x+a ₂)²))}

y ₃ =b ₃+√{square root over ((r ₃ ²−(x−a ₂)²))}

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram illustrating a conventional backlightmodule;

FIG. 2 shows a schematic diagram illustrating the distribution ofbright/dark bands of a conventional light guide plate;

FIG. 3 shows a schematic diagram illustrating an embodiment ofconventional light-diffusing structures;

FIG. 4 shows a schematic diagram illustrating another embodiment ofconventional light-diffusing structures;

FIGS. 5 a and 5 b shows a schematic diagram illustrating a light guidefilm according to an embodiment of the invention;

FIG. 6 a-6 c show schematic diagrams illustrating the various segmentsof the composite lens feature according to an embodiment of theinvention;

FIGS. 7 a and 7 b show schematic diagrams illustrating the lightdiffusing capability of the composite lens feature with a gap betweeneach adjacent feature;

FIG. 8 shows another embodiment of this invention;

FIGS. 9 a and 9 b show the luminance intensity at various distances fromthe light input surface for a circular or arc shaped input feature;

FIGS. 10 a and 10 b show the luminance intensity at various distancesfrom the light input surface for a trapezoidal feature or feature withslanted sides; and

FIGS. 11 a and 11 b show the luminance intensity at various distancesfrom the light input surface according to an embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

A light guide film in accordance with the present invention comprises alight output surface, a light redirecting surface and at least one lightinput surface that joins the light output surface and the lightredirecting surface. The light input surface comprises a plurality ofconcave features consisting of a composite lens array. Each of thecomposite lenses is separated by a gap that is a flat surfaceperpendicular to the light output surface. The composite lenses and gapsare disposed along the light input surface, and extend from the outputsurface to the light redirecting surface. Each of the composite lenseshas a symmetric cross-section consisting of a tip portion comprising acircular tip segment with a first contact angle and a base portioncomprising two tilted circular base segments each with a second contactangle, the second contact angle being greater than the first contactangle and where the radius for each of the two tilted circular basesegments are equal.

According to the above embodiment, the geometrical profile of thecomposite lens allows for comparatively large light deflectingdistances; that is, the composite lens structure has betterlight-diffusing capability. Thus, the distance between the point lightsource and the active area of the display can be shortened, and the darkspots between the point light sources can be minimized, with thebrightness uniformity still being acceptable. The circular tip segmentuniformly distributes the light in front of the discrete light source,typically a light emitting diode (LED). The two tilted circular basesegments uniformly distribute the light between the LEDs. A smoothcurvature of the circular tip segment and tilted circular base segmentsmaximizes the uniformity of the light spatial distribution so that thelight output is uniform. Further, it is also necessary that each twoadjacent composite lens structures have a gap or flat therebetween so agreater degree of deflection on the propagation path of the incidentlight can be achieved to thereby increase the light-diffusing effect.

Referring to FIGS. 5 a and 5 b, a light guide film according to anembodiment of the invention is shown, wherein a planar light guide film12 is used to receive and guide the light from at least one point lightsource (such as LEDs 14 shown in FIG. 5 a). The side surface of thelight guide film 12 next to the LED 14 forms a light input surface 12 a.The top surface of the light guide film 12 that makes an angle with thelight input surface 12 a forms a light-emitting surface 12 b, and thebottom surface opposite the light-emitting surface 12 b forms alight-reflecting surface 12 c. The light-reflecting surface 12 c iscomprised of a plurality of light reflecting structures. The lightemitted from the LED 14 enters the light guide film 12 via the lightinput surface 12 a and propagates inside the light guide film 12. Then,it is guided toward the light-emitting surface 12 b by thelight-reflecting surface 12 c and finally exits the light guide film 12through the light-emitting surface 12 b.

Further, a plurality of concave composite lens structures 16 areserrated on the edge of the light input surface 12 a, with theirlongitudinal directions being parallel to each other and having a gap(G) between each adjacent composite lens structure 16. Referring now toFIGS. 6 a, 6 b and 6 c, the light input surface 12 a, facing the LED 14,of the composite lens structure 16 has a circular tip segment 16 a, andtwo tilted circular base segments 16 b and 16 c, respectively. Thecircular tip segment 16 a of the concave composite lens structure 16 isthe segment furthest from the light input surface 12 a. Although thecomposite lens features for the preferred embodiment of this inventionare disposed in a concave direction on the light input surface, thecomposite lens may also be in a convex direction on the light inputsurface.

The length T₁ is the distance between the intersections of theextensions of a tangent at the top of the circular base segments 16 band 16 c, and a tangent of the circular tip segment 16 a, where thetangent of the circular tip segment 16 a is parallel to the light inputsurface 12 a. The length T₂ is the total width of the circular tipsegment 16 a taken where the circular tip segment 16 a intersects eachthe two circular base segments 16 b and 16 c. Note, T₂ is parallel toT₁. The contact angle A₁ is the contact angle of the circular tipsegment 16 a at the point where the circular tip segment intersects thecircular base segment 16 b. Contact angle A₁ is preferably greater than0.1 degrees and less than or equal to 85 degrees. Referring now to FIG.6 b, the gap G is the distance between each adjacent composite lens.Preferably, the gap G is less than or equal to 0.9 times the pitch P.The pitch P of the linear composite lens array is the distance along thelight input edge which includes the gap G distance and the total width Bof the composite lens. Preferably the pitch P is greater than or equalto 5 micrometers and less than or equal to 1 millimeter (mm) The totalheight H of the feature is measured from the light input edge to thetangent of the circular tip segment 16 a. The total height H of thecomposite lens is greater than or equal to 3 micrometers and less thanor equal to 1 millimeter. The light input surface 12 a will have asurface finish of 10 nanometers to 2 micrometers. The surface finish ofthe concave composite lens structures 16 can be the same or differentthan the gap G portion between the features.

Advantageously, the shape of an XY section of the circular tip segment16 a satisfies the following expression (1):

y ₁ =a ₁+√{square root over ((r ₁ ² −x ²))}  (1)

where the circular tip segment 16 a has a first radius r₁ The firstradius r₁ is defined as the quotient of half the distance T₁ divided bythe tangent of half the contact angle A₁. The value a₁ is defined as thetotal height H minus the radius r₁ of the circular tip segment 16 a. Thevalue x is a value in the direction of the light input surface and ispreferably set within the range of −r₁ x sin(A₁)≦x≦r₁×sin(A₁). The valuey₁ is a value in the light propagation direction.

Referring now to FIG. 6 c, the composite lens structure also comprisestwo tilted circular segments, namely a first circular base segment 16 band a second circular base segment 16 c. Each circular base segmentcomprises two contact angles, a top contact angle and a bottom contactangle. The first circular base segment 16 b has a top contact angle A₂₁and a bottom contact angle A₂₂. The second circular base segment 16 chas a top contact angle A₃₁ and a bottom contact angle A₃₂. The topcontact angle A₂₁ is created by a tangent to the first circular basesegment 16 b at the point where the circular tip segment 16 a and thefirst circular base segment 16 b intersect. The bottom contact angle A₂₂is created by a tangent to the first circular base segment 16 b at thepoint where the first circular base segment 16 b intersects the lightinput surface 12 a. The top contact angle A₂₁ of the first circular basesegment 16 b and the top contact angle A₃₁ of the second circular basesegment 16 c are equal. The bottom contact angle A₂₂ of the firstcircular base segment 16 b and the bottom contact angle A₃₂ of thesecond circular base segment 16 c are equal. The contact angles for eachof the two circular base segments 16 b and 16 c are larger than thecontact angle A₁ of the circular tip segment 16 a.

Advantageously, the shape of an XY section of the circular base segments16 b and 16 c as shown in FIG. 6 c satisfy the following expressions (2and 3) respectively:

y ₂ =b ₂+√{square root over ((r ₂ ²−(x+a ₂)²))}  (2)

y ₃ =b ₃+√{square root over ((r ₃ ²−(x−a ₂)²))}  (3)

Wherein:

r ₂ =H ₂/[cos(A ₂₁)−cos(A ₂₂)]

a ₂=[(T ₂ +B)/2−H ₂×√{square root over ((16r ₂ ²−(B−T ₂)²−4H ₂ ² )/(B−T₂)²+4H ₂ ²))}{square root over ((16r ₂ ²−(B−T ₂)²−4H ₂ ² )/(B−T ₂)²+4H ₂²))}]/2

b ₂=−√{square root over (r ₂ ²−(B/2−a ₂)²)}

B=T ₂ +r ₂×[sin(A ₂₂)−sin(A ₂₁)]+r₃×[sin(A ₃₂)−sin(A₃₁)]

r ₃ =H ₂/[cos(A ₃₁)−cos(A ₃₂)]

a ₃=−[(T ₂ +B)/2−H ₂×√{square root over ((16r ₃ ²−(B−T ₂)²−4H ₂ ²)/(B−T₂)²+4H ₂ ²))}{square root over ((16r ₃ ²−(B−T ₂)²−4H ₂ ²)/(B−T ₂)²+4H ₂²))}]/2

b ₃=−√{square root over (r ₃ ²−(B/2−a ₃)²)}

H ₂ =H−r ₁×[1−cos/(A ₁)]

Thus, the first circular base segment 16 b has a radius r₂ and thesecond circular base segment 16 c has a radius r₃. Referencing FIGS. 6 aand 6 c, the radius r₂ of the first circular base segment 16 b isdefined as the quotient of the height H₂ of the first circular basesegment 16 b divided by the quantity the cosine of the contact angle A₂₁at the top of the first circular base segment 16 b minus the cosine ofthe contact angle A₂₂ at the bottom of the first circular base segment16 b. The height H₂ of the first circular base segment 16 b is equal tothe total height H of the composite lens feature 16 minus the radius r₁of the circular tip segment 16 a times the quantity 1 minus the cosineof contact angle A₁ of the circular tip segment 16 a.

The parameter a₂ is equal to one half the quotient of the quantity ofone half the quantity the total width T₂ of the circular tip segment 16a plus the total width B of the composite lens feature 16 minus thequantity the height H₂ of the first circular base segment 16 b times thesquare root of the quotient of the quantity 16 times the square of theradius r₂ of the first circular base segment 16 b minus the square ofthe quantity of the total width B of the composite lens feature 16 minusthe width T₂ of the circular tip segment 16 a, minus 4 times the squareof the total height H₂ of the first circular base segment 16 b dividedby the square of the quantity of the total width B of the composite lensfeature 16 minus the width T₂ of the circular tip segment 16 a, plus 4times the square of the total height H₂ of the first circular basesegment 16 b.

The parameter b₂ is equal to the negative square root of the quantity ofthe radius r₂ of the first circular base segment 16 b squared minus thequantity one half the total width B of the composite lens feature 16minus the parameter a₂ that quantity squared.

The coordinate x is a value in the direction of the light input surfaceor more specifically in the direction of the total width B of thecomposite lens feature 16 and is preferably set within the range of−B/2≦x≦−T₂/2. The coordinate y₂ is a value in the light propagationdirection.

The total width B of the composite lens feature 16 is equal to the totalwidth T₂ of the circular tip segment 16 a plus the quantity of theradius r₂ of the first circular base segment 16 b times the quantitysine of the contact angle A₂₂ at the bottom of the first circular basesegment 16 b minus the sine of the contact angle A₂₁ at the top of thefirst circular base segment 16 b, plus the quantity of the radius r₃ ofthe second circular base segment 16 c times the quantity sine of thecontact angle A₃₂ at the bottom of the second circular base segment 16 cminus the sine of the contact angle A₃₁ at the top of the secondcircular base segment 16 c. The contact angles for the composite lensfeature can be described where A₂₁=A₃₁, A₂₂=A₃₂ and A₁≦A₂₂, A₂₁.Preferably, A₁≦A₂₂, A₂₁≦85 degrees.

Referencing FIGS. 6 a and 6 c the radius r₃ of the second circular basesegment 16 c is defined as the quotient of the height H₂ of the secondcircular base segment 16 c divided by the quantity the cosine of thecontact angle A₃₁ at the top of the second circular base segment 16 cminus the cosine of the contact angle A₃₂ at the bottom of the secondcircular base segment 16 c. The height H₂ of the second circular basesegment 16 c is equal to the total height H of the composite lensfeature 16 minus the radius r₁ of the circular tip segment 16 a timesthe quantity 1 minus the cosine of contact angle A₁ of the circular tipsegment 16 a.

The parameter a₃ is equal to one half the quotient of the quantity ofone half the quantity the total width T₂ of the circular tip segment 16a plus the total width B of the composite lens feature 16 minus thequantity the height H₂ of the second circular base segment 16 c timesthe square root of the quotient of the quantity 16 times the square ofthe radius r₃ of the second circular base segment 16 c minus the squareof the quantity of the total width B of the composite lens feature 16minus the width T₂ of the circular tip segment 16 a, minus 4 times thesquare of the total height H₂ of the second circular base segment 16 cdivided by the square of the quantity of the total width B of thecomposite lens feature 16 minus the width T₂ of the circular tip segment16 a, plus 4 times the square of the total height H₂ of the secondcircular base segment 16 c.

The parameter b₃ is equal to the negative square root of the quantity ofthe radius r₃ of the second circular base segment 16 c squared minus thequantity one half the total width B of the composite lens feature 16minus the parameter a₃ that quantity squared.

The coordinate x is a value in the direction of the light input surfaceor more specifically in the direction of the total width B of thecomposite lens feature 16 and is preferably set within the range ofT₂/2≦x≦B/2. The coordinate y₂ is a value in the light propagationdirection.

FIG. 7 a is a ray tracing for an array of a single composite lensfeature 16 of this invention illustrating what happens to the light rayswhen the individual composite lens features are disposed on the lightinput surface 12 a in a contiguous manner such that there is no gap Gbetween adjacent composite lenses. FIG. 7 b is a similar ray tracing,but where the individual composite lens feature is separated by a gap Gbetween adjacent features. The gap G is preferably less than or equal to0.9 P where P (as shown in FIG. 6 b) is the pitch of the composite lensfeature on the input surface 12 a. In FIG. 7 a, where the composite lensfeatures are adjacent each other along the input surface, some of thelight rays will experience a secondary light collimation as they arerefracted when they reach the side of the adjacent feature. Thissecondary light collimation detracts from the diffusion capability ofthe composite lens feature 16. In FIG. 7 b, the composite lens featuresare separated by a gap G. The gap allows the light ray to continue in adiffuse manner and thus widens the angle at which the light propagatesin the light guide film. There is minimal secondary light collimationwhen the gap between features is incorporated into the composite lensfeature design. In this way, the wider angle of light helps to mitigatethe hot spots along the input surface of the light guide film.

Referring now to FIG. 8, the light guide film 12 in FIG. 8 shows thecomposite lens features 16 not disposed along the entire input surface12 a. Instead, the composite lens features 16 are disposed along thelight input surface 12 a in the region where the LED 14 light isincident. The luminance uniformity of the system is minimally affectedas the unpatterned region on the light input surface has minimal lightrays in this region.

EXAMPLES

FIG. 9 a shows a portion of the light input surface 32 of a light guidefilm 30 with an arc- or circular-type structure 36. The graph in FIG. 9b illustrates the light intensity for the light guide film 30 atdistances 3.5 mm, 4.5 mm and 5.5 mm from the light input surface 32.FIG. 9 b shows that the localized light intensity decreases as thedistance increases from the light input surface, but there are stillsome hot spots evident at 5.5 mm The arc- or circular-type structuresolution provides some improvement for hot spots but is more effectiveat collimating light in line with the LED than widening the incidenceangle. This is evident in the graph in FIG. 9 b. In FIG. 9 b, the LEDsare located at each of the vertical dotted lines and the lightdistribution is still not leveled out at 5.5 mm into the light guidefilm. It is apparent from the graph in FIG. 9 b that the arc- orcircular-type solution has insufficient diffusion capability.

FIG. 10 a shows a portion of the light input surface 42 of a light guidefilm 40 with a composite lens structure that has flat slanted sides 46.This result would also be applicable to a trapezoidal shaped light inputstructure. The graph in FIG. 10 b illustrates the light intensity forthe light guide film 40 at distances 3.5 mm, 4.5 mm and 5.5 mm from thelight input surface 42. FIG. 10 b shows that the localized lightintensity actually inverts in the area immediately in front of the LEDs,resulting in a dark spot immediately in front of the LEDs. This overallloss of light intensity immediately in front of the LED is due to thefact that the straight slanted walls diffuse the light more readilythrough the sides than through the tip. It is also noted that the shapeof the light intensity profile across the light guide film does notchange significantly as the distance increases from the input surface42.

FIG. 11 a shows a portion of the light input surface 52 of a light guidefilm 50 with the composite lens feature 56 of this invention. Thecomposite lens feature utilizes a circular tip segment and two tiltedcircular base segments. The radius of each of the two tilted circularbase segments is equal. The bottom contact angle of each of the twotilted circular base segments is greater than the contact angle of thecircular tip segment. The circular tip segment uniformly distributes thelight in the area immediately in front of the LED. The two tiltedcircular base segments uniformly distribute the light between the LEDs.The smooth curvatures of the circular tip segment and the two tiltedbase segments maximize the uniformity of the light spatial distributionso the output light is uniform. The graph in Figure llb illustrates thatthe composite lens 56 of the present invention generates uniform lightoutput across the light guide film at distances of 3.5 mm, 4.5 mm and5.5 mm from the input surface 52.

Hence, an improved light guide film is provided with symmetric lightredirecting features to improve light output uniformity withoutsacrificing light input efficiency. Namely, the improved light guidefilm 12 having composite lens structure 16 provides enhanced lightdiffusion in the plane parallel to the light extraction plane and lightreflection plane (top and bottom surfaces), allowing greater lightredistribution between discrete light sources (light traveling outsidethe critical angle of planar un-serrated input edge), so that the lightoutput uniformity is improved. Moreover, the light distribution in theplane perpendicular to the light extraction plane and light reflectionplane (top and bottom surfaces) is minimized, so that the condition ofthe total internal reflection is minimized for the inputted travelinglight.

1. A planar light guide film for a backlight unit having at least onepoint light source, the light guide film comprising: a light inputsurface for receiving light from the point light source; a lightredirecting surface for redirecting light received from the light inputsurface; a light output surface for outputting at least the lightredirected from the light redirecting surface; wherein the light inputsurface further comprises a composite lens structure having a circulartip segment with a first contact angle, and a first and second circularbase segments with a second contact angle, the second contact anglebeing greater than the first contact angle and the second contact anglebeing equal to each other; and wherein the circular tip segmentsatisfies the following equation:y ₁ =a ₁+√{square root over ((r ₁ ² −x ²))} and the circular basesegments satisfies the following equations:y ₂ =b ₂+√{square root over ((r ₂ ²−(x+a ₂)²))}y ₃ =b ₃+√{square root over ((r ₃ ²−(x−a ₂)²))}
 2. The planar lightguide film of claim 1 wherein the composite lens structure has a pitch Pgreater than or equal to 5 micrometers and less than or equal to 1millimeter.
 3. The planar light guide film of claim 2 wherein thecomposite lens structure has a gap G less than or equal to 0.9 times thepitch P.
 4. The planar light guide film of claim 1 wherein the compositelens structure has a total height H greater than 3 micrometers and lessthan or equal to 1 millimeter.
 5. The planar light guide film of claim 1wherein the circular tip segment of the composite lens structure has acontact angle A₁greater than 0.1 degrees and less than or equal to 85degrees.
 6. The planar light guide film of claim 5 wherein the compositelens structure further comprises contact angles A₂₁, A₃₁, A₂₂, and A₃₂wherein A₂₁=A₃₁, A₂₂=A₃₂ and A₁≦A₂₂.
 7. A planar light guide film for abacklight unit having at least one point light source, the light guidefilm comprising: a light input surface for receiving light from thepoint light source; a light redirecting surface for redirecting lightreceived from the light input surface; a light output surface foroutputting at least the light redirected from the light redirectingsurface; wherein the light input surface further comprises a compositelens structure having gaps there between, the lens structure having acircular tip segment with a first contact angle, and a first and secondcircular base segments with a second contact angle, the second contactangle being greater than the first contact angle and the second contactangle being equal to each other; and wherein the circular tip segmentsatisfies the following equation:y ₁ =a ₁+√{square root over ((r ₁ ² −x ²))} and the circular basesegments satisfies the following equations:y ₂ =b ₂+√{square root over ((r ₂ ²−(x+a ₂)²))}y ₃ =b ₃+√{square root over ((r ₃ ²−(x−a ₂)²))}
 8. The planar lightguide film of claim 7 wherein the circular tip segment of the compositelens structure has a contact angle A₁ greater than 0.1 degrees and lessthan or equal to 85 degrees.
 9. The planar light guide film of claim 8wherein the composite lens structure further comprises contact anglesA₂₁, A₃₁, A₂₂, and A₃₂ wherein A₂₁=A₃₁, A₂₂=A₃₂ and A₁≦A₂₂.
 10. A planarlight guide film for a backlight unit having at least one point lightsource, the light guide film comprising: a light input surface forreceiving light from the point light source; a light redirecting surfacefor redirecting light received from the light input surface; a lightoutput surface for outputting at least the light redirected from thelight redirecting surface; wherein the light input surface furthercomprises a serrated lens structure that is provided only where thepoint light source is incident on the light input surface, the lensstructure having a circular tip segment with a first contact angle, anda first and second circular base segments with a second contact angle,the second contact angle being greater than the first contact angle andthe second contact angle being equal to each other; and wherein thecircular tip segment satisfies the following equation:y ₁ =a ₁+√{square root over ((r ₁ ² −x ²))} and the circular basesegments satisfies the following equations:y ₂ =b ₂+√{square root over ((r ₂ ²−(x+a ₂)²))}y ₃ =b ₃+√{square root over ((r ₃ ²−(x−a ₂)²))}